Technical Field
The invention relates to a membrane for a membrane-electrode assembly (MEA) of a fuel cell, comprising two partial membranes, to a membrane-electrode assembly, a fuel cell, and a method for producing a membrane for a membrane-electrode assembly.
Description of the Related Art
Fuel cells use the chemical conversion of a fuel with oxygen into water in order to generate electrical energy. For this purpose, fuel cells contain the so-called membrane-electrode assembly (MEA) as a core component, which is an arrangement of an ion-conducting (usually proton-conducting) membrane and of a catalytic electrode (anode and cathode) respectively arranged on both sides of the membrane. The electrodes generally comprise supported precious metals, in particular platinum. Depending on the design, the arrangement is sometimes also called catalyst-coated membrane (CCM). In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane-electrode assembly on the sides of the electrodes facing away from the membrane. Generally, the fuel cell is formed by a plurality of individual MEA cells which are arranged in the stack and the electrical power outputs of which add up. Bipolar plates (also called flow field plates or separator plates), which ensure a supply of the individual cells with the operating media, i.e., the reactants, and which are usually also used for cooling, are generally arranged between the individual membrane-electrode assemblies. In addition, the bipolar plates also ensure an electrically conductive contact to the membrane-electrode assemblies.
During operation of the fuel cell, the fuel (anode operating medium), particularly hydrogen H2 or a gas mixture containing hydrogen, is supplied to the anode via an open flow field of the bipolar plate on the anode side, where electrochemical oxidation of H2 to protons H+ with loss of electrons takes place (H2→2H++2 e−). Protons are transported (in a water-bound or water-free manner) from the anode chamber into the cathode chamber via the electrolyte or membrane that separates and electrically insulates the reaction chambers in a gas-tight manner from each other. The electrons provided at the anode are guided to the cathode via an electrical line. The cathode receives, as cathode operating medium, oxygen or a gas mixture containing oxygen (such as air) via an open flow field of the bipolar plate on the cathode side so that a reduction of O2 to O2− with gain of electrons takes place (½O2+2 e−→O2−). At the same time, the oxygen anions react in the cathode chamber with the protons transported across the membrane to form water (O2−+2 H+→H2O).
The fuel cell stack is supplied with its operating media, that is the anode operating gas (hydrogen for example), the cathode operating gas (air for example), and the coolant by means of the main supply channels, which run through the stack in its entire stack direction and from which the operating media are supplied to the individual cells via the bipolar plates. There are at least two such main supply channels available for each operating medium, namely one for supplying and one for discharging the respective operating medium.
For the stable operation of a fuel-cell system, a specific water circuit within a membrane-electrode assembly by various measures is, among other things, an important criterion since the membrane-electrode assembly may neither dry out nor have high humidity. The water circuit relates to the water formed in the cathode chamber and also to externally supplied water, wherein external humidification of the membrane-electrode assembly may, where applicable, be dispensed with as a result of a specific water transport to the anode or cathode. Furthermore, the risk of damage to the electrode by fuel starvation (lack of reactant) must be minimized, e.g., by water discharge from the region of the anode.
For guiding water within the membrane-electrode assembly, it is known to use thinner than unusual membranes, which allow an easy water exchange between the anode and cathode, wherein thinner membranes however decrease the efficiency of the cell and are more unstable mechanically.
It is also known to provide water for humidification of the membrane by specific recombination of reaction gases. For example, DE 199 17 812 C2 describes a membrane-electrode assembly for a fuel cell, in which membrane-electrode assembly is provided a catalyst layer which is localized within the membrane and at which a recombination takes place in order to generate water. The membrane may consist of two partial membranes which are made of Nafion® and which are stacked on top of each other after the catalyst layer is arranged.
Fuel-cell membranes are otherwise generally homogeneously made of a chemically and physically uniform polymer electrolyte, wherein a porous carrier film, e.g., based on e-PTFE (expanded polytetrafluoroethylene), can be enclosed.
Used as material for the polymer electrolyte are in many cases perfluorosulfonic acid polymers (PFSA membranes) or even sulfonated hydrocarbon polymers (HC membranes). These polymer electrolytes are characterized by their ion exchange capacity (IEC), which depends on the concentration of sulfonic acid groups in the polymer.
The PFSA membranes have the advantage of a higher chemical stability compared to the hydrocarbon membranes, in particular with respect to the oxygen radicals preferably formed at the fuel-cell cathode, whereas the hydrocarbon membranes have the advantage of lower gas permeation with the same thickness of the membrane in comparison to the perfluorosulfonic acid membranes and of more cost-effective base materials.